#1 Tip for any creative or technical achiever at that age: Build up your network. Even in mid-size cities in the US there are inter-scholastic robotic battle arenas, programming competitions, e-sports leagues. Some compete up to the national or international levels. If they can gain even 300 Github or Discord followers by the time they graduate, that forms a pool of friends and family with which to draw on. Once it gets out that they have web and app development skills, requests for consults will usually come to them. In rare cases I have even seen student run VR clubs attract business from local merchants. Although in those cases it's usually around a major tech hub.

#2 point is to always say Yes to things. You've probably encountered a similar scenario. Student meets friend's dad at a BBQ. The dad says he wants a way for his buddies to keep track of their golf scores on their phones. Student's instinct is "But I'm into AI and Robotics, I don't know the first thing about hitting the links!" Train them to adjust their attitude to servicing the clients needs. Even if it requires an all night research session on Wikipedia. Or actually taking a lesson from a pro at the local country club.

Best of Luck!

OP here, providing the additional backstory:

15 years ago, when I started freelancing, I was a full-stack developer specializing in Drupal. That is, I ran my own servers and websites using Drupal. I had spent a long time (a few years before that) on the Drupal forums answering people's questions for free, because it was fun to do, like solving a logic puzzle. I never even though about getting paid for it, because i didn't have a lot of business confidence and didn't even have a CS degree (or any degree, for that matter!). The Drupal forums also had a "Paid Drupal Services" section. One day, I read a job posting for a small (~$75) job, and realized that I knew how to solve it very quickly. I had no references, but when I applied, I pointed to my public forum history, which showed me solving people's problems. I got the job, had it fixed in 15 minutes, and received payment, all in less than 5 hours start-to-finish. Rinse and repeat. In fact, I never provided references... I just pointed to my forum (community) activity, and I kept getting jobs. Eventually, some of those jobs were from small Drupal shops who were subcontracting parts of larger projects, and once they had a good experience with me, they kept forwarding me more and more jobs. Eventually, the freelancing connections lead to a salaried job (last year of my bachelor's and through my master's degree).

Why don't I think that this will work now? Well, specialized communities aren't that common. Now we have Stack Overflow, and the simple questions are already answered. I don't know of a "Paid Services" forum like Drupal had (even theirs has very little activity now!). Basically, I don't know how a modern student can get started in freelancing, because I don't know the modern resources that they would need. I stopped freelancing when I started my Ph.D., so even my old connections are (mostly) out of business. Furthermore, I'm not looking to produce more Drupal devs (although that's not a bad thing), but these students mostly know Python and C++.

As someone who's been freelancing over 5 years, this is pretty close. The usual story is "someone asked me to do it and I asked for money and people kept giving more money."

Let them know to avoid freelancer and elance.

Also what I have been trying: building stuff, waiting clients. Doesn't work.